1. Field of the Invention
The present invention relates to an optical deflector, and in particular to an optical deflector which includes an angular-displacement detecting portion.
2. Description of the Related Art
In recent years, an optical deflector which is manufactured by using a micro machine technology in which a semiconductor manufacturing technology is applied has been drawing attention. For example, in Japanese Patent Application Laid-open Publication No. Hei 5-119280, an optical deflector which is used in a scanner of a projection display and a laser printer, has been disclosed.
FIG. 12 denotes a structure of an optical deflector disclosed in Japanese Patent Application Laid-open Publication No. Hei 5-119280. The optical deflector includes a mirror portion 9 which has a mirror surface 10 having both ends thereof supported by torsion bars 8, and electrode portions 12a and 12b which are provided on a substrate 7 facing a side of a rear surface of the mirror surface 10. There is an electrostatic-attraction force acting between the electrodes 12a and 12b, and the rear surface of the mirror surface 10. Due to the electrostatic-attraction force an angle of inclination of the mirror surface 10 is changed. Strain gauges R1, R2, R3, and R4 are formed on the torsion bars 8. A change in a resistance of the strain gauges R1, R2, R3, and R4 is measured. An angle of inclination θ of the mirror surface 10 is detected from the change in the value of resistance. A unit which controls the angle of inclination of the mirror surface controls the angle of inclination θ of the mirror surface 10 by using the detected value of the angle of inclination θ. This enables a closed loop control. As a result, harmful vibrations are eliminated and a contact of the mirror section with the electrode is reduced.
In the structure denoted in FIG. 12, the strain gauges R1 and R2 are disposed on one of the torsion bars and the strain gauges R3 and R4 are disposed on the other torsion bar. If the strain gauges which form the Wheatston bridge circuits are at positions away from each other, it is susceptible to influence of a surrounding temperature distribution. For example, a case in which there is a driving circuit near the strain gauges R1 and R2 is taken into consideration. In this case, the strain gauges R1 and R2 are susceptible to a thermal influence from the driving circuit. For this reason, a temperature drift which differs from the temperature drift in the strain gauges R3 and R4 is developed in the strain gauges R1 and R2. As a result of this, there is deterioration in accuracy of detection of an angular displacement (inclination) of the mirror surface, which is an issue.